The present invention relates to an ATM (Asynchronous Transfer Mode) transfer system of data packets for transferring low bit-rate and variable-length data packets such as voice packets making use of an ATM transfer network.
Conventionally, data packets such as voice packets are transferred on ATM cells by way of a VC (Virtual Channel) established between a packet transmitter and a packet receiver when the data packets are transmitted from a terminal to a terminal.
When data packets are to be transmitted from a terminal to a plurality of terminals for a broadcast communication, data packets are duplicated at an ATM layer. An example of the broadcast communication method is disclosed in a Japanese patent application laid open as a Provisional Publication No. 349737/'92, wherein each of ATM cells input to an ATM exchange is duplicated into N ATM cells when the data packets transmitted from a packet transmitter are to be transferred to N (N being an integer more than one) packet receivers regardless of data packets loaded on the ATM cells. By rewriting header information of every duplicated ATM cell with header information indicating each corresponding destination by the ATM exchange at the ATM layer, each of the plurality of ATM cells having the same data is output to be delivered to each of the plurality of packet receivers.
In conventional ATM transfer systems as above described, each one data packet is transmitted loaded on one ATM cell, using ATM circuits with low efficiency. It is because a part of the ATM cell is left useless occupying communication capacity of the ATM network in vain when a data packet such as a voice packet is loaded on the ATM cell having a fixed data length, since the low bit-rate and variable-length coding of the voice packet has been remarkably progressed along with progress of the voice data compression technique. Furthermore, this inefficiency accompanying the low bit-rate and variable-length coding of the voice packet is also duplicated in the above broadcast method wherein the same number of ATM cells to the number of destinations are all duplicated at the ATM layer.
For improving the transfer efficiency of the ATM network, multi-loading of the ATM cell is proposed recently, wherein data packets having different destinations, designated by AAL (ATM Adaptation Layer) header information added to each of the data packets, are multi-loaded on the same ATM cell, which is duplicated at a transmitter-side exchange to be sent out to different receiver-side exchanges. multi-loading refers to the process of multiplexing and packing the data packets on the ATM cell. thus improving efficiency by utilizing more of the communication channel.
However, when the multi-loaded ATM cell is duplicated at the ATM layer, in such a way as performed in the above broadcast method, according to fixed destinations registered when the call is established, there is still left certain inefficiency. This is because data packets such as voice packets may not be generated regularly, and ATM cells having no concerning data packet may be duplicated and transferred in vain to some receiver-side exchanges when data packets concerning to the receiver-side exchanges are not generated.